As is known, ethylene or in general alpha-olefins can be polymerized by the Ziegler low-pressure method.
To this end, use is made of catalysts generally comprising a compound of elements in groups IV to VI in the periodic system (compounds of transition metals) mixed with an organometallic compound or hydride of elements in groups I to III of the periodic system, operating in suspension or in solution or in the gas phase.
In other known catalysts, the transition metal component is fixed on an organic or inorganic solid substrate, and physically and/or chemically treated if required. The following are examples of solid substrates: oxygenated compounds of divalent metals (e.g. oxides or oxygenated or carboxylated inorganic salts) or hydroxychlorides or chlorides of divalent metals.
One particular substrate for the transition metal compound is the solid free-flowing product obtained by spray-drying a solution of magnesium chloride in ethanol as described e.g. in U.S. Pat. Nos. 4,421,674 and 4,481,342. The resulting substrates, which have a certain content of alcoholic hydroxyls, react esily with titanium tetrachloride to give solid catalyst compounds having high activity in the polymerization of ethylene and the copolymerization of ethylene with an alpha-olefin.
One characteristic of the catalyst components described hereinbefore is the relatively low content of titanium, typically of the order of 5% by weight expressed as metal. It is therefore desirable to provide catalyst components having a titanium content which is higher and in active form, so as to obtain improved performances in the polymerization of ethylene or the copolymerization of ethylene with alpha-olefins.
In industry, a need has also been felt for catalysts capable of producing ethylene polymers or ethylene copolymers with alpha-olefins having restricted distributions of molecular weights and a desirable group of characteristics with regard to the melt index and shear sensitivity and an extremely low content of fines. These ethylene polymers are particularly suitable for injection-moulding.
It has now been found that these requirements can be met by the catalyst component and the catalyst according to the invention.